The Ascension frigatebird, a threatened species, may have been saved from the brink of extinction thanks to a cat eradication programme.

The Ascension frigatebird is classified as Vulnerable on the IUCN Red List

On the edge

Islanders and conservationists alike are celebrating the recent news that the Ascension frigatebird, one of the world’s rarest seabirds, has returned to breed on the remote Ascension Island in the South Atlantic, 150 years after its colony on the island was wiped out by feral cats.

Until recently, this species’ only breeding ground was a rocky outcrop known as Boatswain Bird Island, off Ascension’s east coast, where a small colony of around 10,000 birds survived. Given its highly restricted range and limited numbers, the Ascension frigatebird was considered to be extremely susceptible to outbreaks of disease and the effects of oil spills.

However, ornithologists have recently recorded two nests containing eggs and being guarded by Ascension frigatebirds on the island itself, an event which has not occurred since Charles Darwin visited the island in the early 19th century. The encouraging news gives hope that this threatened bird may still be brought back from the brink of extinction.

Boatswain Bird Island which, until recently, was the sole nesting ground of the Ascension frigatebird

Rampant rodents and feline foes

In the 19th century, Ascension Island, one of the UK Overseas Territories, was home to more than 20 million seabirds, including black noddies, brown noddies, masked boobies and Ascension frigatebirds. Of these, the frigatebird is considered to be of particular importance, as it is endemic to the island, being found nowhere else on Earth.

However, along with human settlers came more than 1,800 rats, which were accidentally introduced to the remote island and were the cause of many chick deaths. As a control measure, cats were imported in the hope that they would kill off, or at least stabilise, the rat population, but this plan backfired and the cats themselves began killing bird chicks.

“By the time Darwin visited the island in 1836, there were only a few frigatebirds left and the last few were killed off not long after he left,” said Clare Stringer of the RSPB, an organisation which has played a key role in re-establishing the frigatebird population on Ascension.

The masked booby is another of Ascension’s fascinating bird species

Eradication

In response to the continued decline of the Ascension frigatebird, as well as that of other bird species nesting on the island, the RSPB launched a cat eradication programme in 2002, funded by the Foreign Office. Despite sounding simple, the programme proved to be rather challenging.

“It was slightly tricky,” said Ms Stringer. “We had to avoid killing islanders’ pet cats and kill only feral animals. Owners were told to collar and microchip their pets. Then traps were laid and feral cats caught in these were put down.”

Four years later in 2006, Ascension Island was officially declared to be free of wild cats. Since then, the island’s conservation officers have been keeping a close eye out for signs of the frigatebird’s return, and were delighted to have discovered two nests so soon after the completion of the cat eradication programme.

“It has taken six years to get frigatebirds to start to recolonise the island since we got rid of the feral cats and frankly it could have taken much longer,” said Derren Fox, one of Ascension’s conservation officers who, along with fellow conservation officer Stedson Stroud, has been carefully monitoring the island. “We now have two nests being tended by parent birds and that should encourage a lot more to settle here in future.”

Immature Ascension frigatebirds sunning themselves

Overwhelming outcome

The £500,000 project has taken several years and much hard work to complete, but its success and the return of the Ascension frigatebird much sooner than expected has shown it to be worthwhile. It has also demonstrated that, when it comes to conservation, perseverance is key.

“We are absolutely overwhelmed,” said Mr Fox. “We thought it would take decades for the Ascension frigate to come back and breed after we had got rid of the island’s feral cats. But we have already succeeded after only a few years. This suggests we have a real chance of saving the Ascension frigate.”

It is hoped that, through its success, this project has paved the way for further similar schemes to save other species at risk from feral animals, including species on Montserrat, Gough Island, and South Georgia, which are all under threat from invasions of rats, mice and other predators.